1. Field of the Invention
The present invention relates to a direct fuel injection engine control system for controlling a direct fuel injection engine in which fuel is directly injected into a combustion chamber. More particularly, the present invention relates to a direct fuel injection engine control system suitable for controlling a direct fuel injection engine that includes a valve varying mechanism for introducing burned gas into a cylinder.
2. Description of the Related Art
JP,A 10-205362, for example, discloses one control system for a conventional direct fuel injection engine. This disclosed system includes a valve varying mechanism for varying an overlap between intake and exhaust so that burned gas in an exhaust port is introduced to a combustion chamber. The temperature in a cylinder is thereby increased to accelerate vaporization of injected fuel for reducing HC (hydrocarbon). The disclosed system is further intended to reduce NOx (nitrogen oxides) based on the effect of exhaust return. According to another known control system disclosed in JP,A 11-294207, for example, a valve varying mechanism is operated to maintain an intake valve on one side closed during the intake stroke and keeps an exhaust valve on one side open during the intake stroke so that a combustion gas area is produced substantially in a half of a cylinder.
However, the control system disclosed in JP,A 10-205362 has the following problems. Because a swirl flow is formed in the combustion chamber by a swirl valve, the burned gas introduced from the exhaust port is forced to spread in the cylinder by a strong swirl flow and to mix with fresh air. Therefore, combustion stability is reduced in an operating range under low load. Further, because this related art is intended to raise the temperature in the entirety of the cylinder, the effect of accelerating fuel vaporization at a spray tip of the injected fuel is low, although the fuel vaporization at the spray tip greatly contributes to a reduction of HC.
Also, the control system disclosed in JP,A 11-294207 has the following problems. Because fuel is injected toward fresh air to improve combustion stability, restrictions occur in arrangement of a fuel injector and direction of fuel injection. Further, because a fuel spray is locally formed in one side of the cylinder, it is apt to reach a cylinder wall more quickly. Therefore, the emission rate of HC is increased at the injection timing near the compression top dead center at which a sufficient vaporization time cannot be obtained.
Accordingly, it is an object of the present invention to provide a control system for a direct fuel injection engine employing a valve varying mechanism, which can suppress mixing of burned gas and fresh air to maintain the combustion stability, can accelerate vaporization of sprayed fuel at the beginning of fuel injection to reduce the emission rate of HC, and can realize a reduction of NOx based on the effect of exhaust return.
(1) To achieve the above object, the present invention provides a control system for a direct fuel injection engine comprising a fuel injector for directly injecting fuel into a combustion chamber, and a valve varying mechanism unit for variably controlling the opening/closing timing of each of an intake valve and/or an exhaust valve, the valve varying mechanism unit being operated to introduce burned gas from an exhaust port into the combustion chamber and to inject the fuel during a compression stroke for effectuating stratified charge combustion, wherein the valve varying mechanism unit is controlled to provide a phase difference in the opening timing of two left and right valves constituting the intake valve and/or the exhaust valve, thereby generating a flow of the burned gas along a cylinder wall.
With those features, mixing of burned gas and fresh air can be suppressed, and therefore the combustion stability can be maintained. Vaporization of sprayed fuel at the beginning of fuel injection can be accelerated, and therefore the emission rate of HC can be reduced. Further, a reduction of NOx can be realized based on the effect of exhaust return.
(2) In above (1), preferably, the valve varying mechanism unit controls a valve overlap between the intake valve and the exhaust valve by varying the opening/closing timing of one of two left and right valves constituting the intake valve, and/or varying the opening/closing timing of one of two left and right valves constituting the exhaust valve.
(3) In above (1), preferably, the valve varying mechanism unit controls a valve overlap between the intake valve and the exhaust valve by providing a phase difference in the opening timing between two left and right valves constituting the intake valve beforehand, providing a phase difference in the opening timing between two left and right valves constituting the exhaust valve beforehand, and varying the opening/closing timing of the intake valve and the exhaust valve.
(4) In above (1), preferably, the valve varying mechanism unit varies the opening/closing timing of one of two left and right valves constituting the intake valve to increase a valve overlap between the intake valve and the exhaust valve, and then holds one of two left and right valves constituting the exhaust valve to be open during a period in which the intake valve is opened, thereby increasing the amount of burned gas introduced.
(5) In above (1), preferably, a valve overlap between the intake valve and the exhaust valve is controlled by varying the opening/closing timing of one of two left and right valves constituting the intake valve, and/or increasing a valve lift of the exhaust valve.
(6) In above (1), preferably, the fuel injector is a high-penetrating injector for injecting the fuel in a spray form with initial spraying or deflected spraying or spray-lead spraying.
(7) In above (6), preferably, the fuel injector injects the fuel twice in the operation with stratified charge combustion.
(8) In above (1), preferably, the control system for the direct fuel injection engine further comprises a piston having a recess formed in a crown surface of the piston to maintain a tumble flow during the compression stroke, and having a swirl guide formed therein on the exhaust side opposite to the recess.
(9) In above (1), preferably, the injecting direction of the fuel injector is aligned with the direction of the burned gas flow along the cylinder wall.